An apparatus has been known in which the quantity of bone mineral of a living body can be measured by measuring the degree of transmission of a low-energy gamma ray beam. Such apparatus is described in "A Bone Density Measuring Apparatus," the Medical Radiation Apparatus Technical Handbook, published by the Japan Industries Association of Radiation Apparatus, Apr. 1, 1983. This bone density measuring apparatus uses a shielded ray source for emitting gamma rays in the form of a beam. The ray source and a scintillation detector are separated from each other by an interval of about 15 cm. A portion to be measured is interposed between the ray source and the scintillation detector and the portion to be measured is moved (or scanned) perpendicularly to the gamma rays.
Analog calculations are performed on the basis of the counted number of the gamma rays transmitted through the portion to be measured at every measuring position so as to obtain a value of weight per unit length of the bone in the portion to be measured. There are two methods for measuring the density of a bone or the like: One is a so-called "underwater single ray-source method" in which a portion to be measured is covered with a water bag to make the thickness of material other than the bone a predetermined value and measurement is made by using a single ray-source. The other method is the "aerial two ray-source method" in which a portion to be measured is put in the air and measured by using two ray-sources. The apparatus previously available on the market is arranged so that measurement can be made by either one of the methods. The bone density measuring apparatus includes a scanning device incorporated with a low-energy gamma ray beam generating portion (.sup.125 I 50 mCi and .sup.241 Am 45 mCi being generally used), a special scintillation detector, and a counting device, etc. The final value obtained by the apparatus is a numerical value in proportion to the weight per unit length of the bone.
In carrying out measurement of bone density of a subject by using the above-mentioned apparatus, it is standard to measure bone density along a straight line of a body of the subject, and, therefore, the irradiation with gamma rays must be repeated many times when bone density of a certain portion is to be measured. Accordingly, it has taken several tens of minutes to measure, for example, one of the vertebrae of a man.